1. Field of the Invention
The present invention relates to a workpiece carrier for holding a workpiece such as a semiconductor wafer while the workpiece is being polished to make a surface of the workpiece to a flat mirror finish, and-a polishing apparatus having such a workpiece carrier.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical polishing.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains (or material) is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated.
If the relative pressure between the semiconductor wafer being polished and the polishing cloth is not uniform over the entire surface of the semiconductor wafer, then the semiconductor wafer tends to be locally polished excessively or insufficiently depending on the applied pressure.
FIG. 6 of the accompanying drawings shows a conventional polishing apparatus. As shown in FIG. 6, a top ring drive shaft 51 has on its lower end a spherical portion 52 which is received in a spherical seat recess 55 defined in an upper surface of a top ring 54 which holds a semiconductor wafer 53 to be polished. The top ring 54 is thus tiltable with respect to the top ring drive shaft 51 so that the top ring 54 follows automatically any possible inclinations of a turntable 56 beneath the top ring 54. The tiltable top ring 54 allows its wafer holding surface 54a to be kept parallel to the upper surface of the turntable 56 for uniformizing the relative pressure between the semiconductor wafer 53 and a polishing cloth 57 attached to the upper surface of the turntable 56 over the entire surface of the semiconductor wafer 53.
According to another proposed polishing apparatus, the top ring drive shaft and the spherical portion are separate from each other and include a top ring drive shaft and a spherical bearing comprising a ball, and the spherical bearing is interposed between the top ring drive shaft and the top ring (see Japanese laid-open patent publication No. 6-198561).
In the polishing apparatus shown in FIG. 6, while the semiconductor wafer 53 is being polished, the top ring drive shaft 51 applies a pressing force F through the top ring 54 to the semiconductor wafer 53, thus developing a frictional force xcexcF (xcexc: coefficient of friction) on the surface of the semiconductor wafer 53 slidingly held against the polishing cloth 57. The frictional force xcexcF produces a rotating moment M=xcexcFH which tends to tilt the top ring 54 depending on the height H of the center O of the spherical portion 52 from the lower surface of the semiconductor wafer 53 slidingly held against the polishing cloth 57. Because of the rotating moment M, the entire lower surface of the semiconductor wafer 53 cannot uniformly be pressed against the polishing cloth 57. In order to make the moment M zero, it is necessary to make the height H of the center of the spherical portion 52 zero. To meet this requirement, there has been proposed a polishing apparatus having a spherical bearing whose tilting center is positioned on the surface of the semiconductor wafer that is slidingly held against the polishing cloth.
The spherical bearing of the above mentioned proposed polishing apparatus has a convex spherical surface of relatively large area disposed on the upper surface side of the top ring and a concave spherical surface disposed on the lower end side of the top ring drive shaft and held in sliding contact with the convex spherical surface. The top ring is tiltable with respect to the top ring drive shaft due to sliding contact between the convex spherical surface and the concave spherical surface. Because of the sliding contact between the convex and concave spherical surfaces, the top ring cannot follow quickly and smoothly the inclinations of the turntable. Consequently, the wafer holding surface of the top ring and the surface of the turntable may be brought out of parallelism with each other, thus tending to cause the semiconductor wafer to be polished while the semiconductor wafer is being tilted with respect to the polishing cloth.
Another problem is that the convex and concave spherical surfaces of the spherical bearing need to be machined to accurate radii of curvature in order to make the spherical bearing function properly.
It is therefore an object of the present invention to provide a workpiece carrier which is capable of allowing a top ring to quickly and smoothly follow possible movements (inclinations) of the upper surface of a turntable, thereby keeping a workpiece holding surface of the top ring in parellelism with the upper surface of the turntable.
Another object of the present invention is to provide a polishing apparatus having such a workpiece carrier.
According to the present invention, there is provided a workpiece carrier for holding a workpiece to be polished and pressing the workpiece against a polishing surface on a turntable, comprising a top ring body for holding the workpiece, a drive shaft for rotating the top ring body and moving the top ring body toward the turntable to press the workpiece against the polishing surface, and a universal joint for transmitting a pressing force from the drive shaft to the top ring body while allowing the drive shaft and top ring body to be tilted respective to each other, the universal joint comprising two members having curved surfaces formed along arcs having a predetermined radius of curvature from a center positioned on a surface of the workpiece which is held in contact with the polishing surface on the turntable, and at least four rolling elements held in rolling contact with the curved surfaces, wherein at least two of the rolling elements are held in rolling contact with the respective curved surfaces to allow the top ring body to be tilted relative to the drive shaft about a point positioned on the surface of the workpiece which is held in contact with the polishing surface on the turntable.
According to the present invention, there is also provided a polishing apparatus for a workpiece, comprising a turntable having a polishing surface thereon, a top ring body for holding the workpiece, a drive shaft for rotating the top ring body and moving the top ring body toward the turntable to press the workpiece against the polishing surface, and a universal joint for transmitting a pressing force from the drive shaft to the top ring body while allowing the draft shaft and the top ring body to be tilted relative to each other, the universal joint comprising two members having curved surfaces formed along arcs having a predetermined radius of curvature from a center positioned on a surface of the workpiece which is held in contact with the polishing surface on the turntable, and at least four rolling elements held in rolling contact with the curved surfaces, wherein at least two of the rolling elements are held in rolling contact with the respective curved surfaces to allow the top ring body to be tilted relative to the drive shaft about a point positioned on surface of the workpiece which is held in contact with the polishing surface on the turntable.
According to the present invention, since a moment which is caused by a frictional force acting on the surface to be polished of the workpiece during polishing and causes the top ring to be tilted is made zero, a workpiece holding surface of the top ring can be kept parallel to the upper surface of the turntable, thereby allowing the workpiece to be polished highly accurately. When the top ring is tilted to follow any possible inclinations of the upper surface of the turntable, the two members which perform the relative motion move relative to each other in accordance with rolling contact, rather than sliding contact, of the rolling elements. As a consequence, the top ring can quickly and smoothly follow any possible movements of the upper surface of the turntable.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.